Aluminum is a very desirable coating material, particularly for oxidizable metals. The surface of aluminum exposed to air rapidly develops an oxide film that protects the remainder of the aluminum from oxidation. Hence, aluminum films may be used to protect the substrate material from oxidation.
In addition to its use as a protective coating, aluminum is also suitable as a conductor in thin film electrical circuits and as a conductor and/or dopant in semiconductor devices such as Josephson junction devices and gallium arsenide or other III-V type semiconductor devices.
Aluminum films are typically formed by means of vacuum evaporation or sputtering, both of which require relatively high vacuums. In addition, substrate heating is also often needed. When operating at elevated temperatures, detrimental effects may occur to the substrate especially when dealing with semiconductors such as gallium arsenide. This problem is especially apparent when the method of deposition involves thermal decomposition of organo-aluminum compounds. Furthermore, where extremely fine line deposition of the aluminum may be required, thermal decomposition techniques require the use of a laser or other concentrated heat source which must be scanned in the desired pattern to be produced and its energy must be adjusted such that the underlying substrate is not damaged by the laser. This can be a difficult task. Alternatively when forming patterns by means of vacuum evaporation or sputtering, contact masks, e.g., photoresists, must be applied to the substrate surface.
U.S. Pat. No. 3,271,180, issued to P. White describes a radiation-stimulated process for forming metal deposits of tin, lead, germanium, silicon, cadmium and the like by u.v. stimulated decomposition of the metal-alkyl complexes such as the metal tetramethyl or tetraethyl complexes. In accordance with this method, decomposition of the compounds by radiation occurs at temperatures between 200.degree. C. and 300.degree. C. as compared to thermal decomposition of the same materials which takes place at about 600.degree. C. Similar processes were described by D. J. Ehrlich et al. in an article appearing in the IEEE, Journal of Quantum Electronics, Vol. QE-16, No. 11, Nov. 1980 wherein laser photodeposition was employed to deposit metal films from metal-alkyl compounds. Here, the use of trimethyl aluminum was specifically set forth. However, the rate of aluminum deposition was reported as being extremely slow.
In U.S. Pat. No. 3,375,129, there is described a thermal decomposition method for the formation of aluminum films employing thermally decomposable vapors of an amine complex of aluminum hydride. These are the same compounds employed by applicants herein for photodecomposition. It is taught therein that it is preferred to employ an amine complex of the plating agent which contains, as the amine constitutent thereof, a tertiary alkylamine having from 1 to 6 carbon atoms. These are said to be preferred since they pyrolize smoothly and cleanly to leave a deposit having a high degree of purity. Trimethylamine and bistrimethylamine complexes of aluminum hydride are said to be especially preferred because of their volatility and stability. In processing such trialkylamine complexes of aluminum hydride it is stated that best results are obtained with temperatures in the range of about 120.degree. C. to about 250.degree. C. As previously stated, it is difficult to employ a thermal technique for the deposition of aluminum where fine patterns are desired since it is difficult to confine the heat to the area required for the pattern. Photostimulated deposition, however, allows one to operate through a non-contact photomask or direct focusing technique mask that the stimulating radiation strikes only the areas in which pattern formulation is desired.
It should be noted that while a given compound may be known to be useful for the production of a metal by thermal decomposition, it is not necessarily suitable for the production of that metal by u.v. stimulated decomposition.